1. Field of the Invention
The present invention relates generally to methods of cementing and low density foamed fly ash cement compositions.
2. Description of the Prior Art
In general cementing operations wherein hard substantially impermeable cementitious masses are formed in desired locations, hydraulic cement compositions have heretofore been utilized which contain hydraulic cements such as Portland cements, gypsum cements, silica cements and the like along with water and other additives. In primary well cementing, i.e., the forming of a cement sheath in the annular space between pipe disposed in a well bore and the walls of the well bore, a pumpable hydraulic cement composition is introduced into the annular space and the cement composition is permitted to set therein. The resulting cement sheath provides physical support and positioning to the pipe in the well bore and prevents undesirable fluid migration between subterranean zones and formations penetrated by the well bore.
In some locations, the subterranean zones or formations into or through which wells are drilled have high permeabilities and low compressive and tensile strengths. As a result, the resistances of the zones or formations to shear are low and they have low fracture gradients. When a well fluid such as a hydraulic cement composition is introduced into the well bore penetrating such a subterranean zone or formation, the hydrostatic pressure exerted on the walls of the well bore can exceed the fracture gradient of the zone or formation and cause fractures to be formed in the zone or formation into which the cement composition is lost.
While lightweight cement compositions have been developed and used heretofore, subterranean zones or formations are still encountered which have fracture gradients too low for even the lightweight cement compositions to be utilized without fracturing the formation and the occurrence of loss circulation problems. Also, the lightweight cement compositions utilized heretofore have often not had sufficient compressive, tensile and bond strengths upon setting.
Thus, there are continuing needs for improved low density cement compositions for use in general cementing as well as in cementing in subterranean zones or formations having very low fracture gradients.